Noise sensitive signals are usually transmitted as a differential pair. The two signals of the differential pair are affected substantially in the same manner by noise and, therefore, taking the difference between the two signals at their destination may cancel a significant portion of the noise added to the signals during transmission.
If the differential pair signals are transmitted over long transmission lines, the signals may degrade due to noise caused by the parasitic series resistance, inductance, and coupling capacitance in the transmission lines. These parasitic elements attenuate high-frequency signal components more than low-frequency signal components and thereby cause a “smearing” or degradation of the waveform of the signals. If the transmission lines are sufficiently long, the degradation may cause the signals to be completely indecipherable by the time they reach the end of the transmission lines.
To overcome this degradation, repeaters may be inserted along the transmission line at regular intervals. FIG. 1 shows multiple differential repeaters 100 used to propagate differential inputs INA and INB along a differential transmission line 105. A termination block 110 is connected to the end of the transmission line 105 and may be optionally used to the end of the transmission line 105 and may be optionally used for interfacing the received differential signal with a receiving circuit (not shown).
FIG. 2 shows an example of a differential amplifier 200 that may be used as a differential repeater (e.g., 100 in transmission line 105). The differential amplifier 200 includes a DC current source 205, two output resistors 210, and two n-type transistors 215. The differential inputs, INA and INB, are amplified by the differential amplifier 200 which outputs them at OUTA and OUTB. However, the use of the differential amplifier 200 as a repeater suffers from several drawbacks.
First, the differential amplifier 200 exhibits high power dissipation caused by the constant current consumption of the DC current source 205. When multiple differential amplifiers 200 are used to drive transmission lines the resulting power dissipation worsens and may become intolerable.
Second, the differential amplifier 200 exhibits low-drive capability caused by the two output resistors 210 forming a low-pass filter with the transmission line capacitance. The attenuation of high-frequency signal components caused by the low pass filter may be lessened by decreasing the resistance of the output resistors 210. However, decreasing the resistance of resistors 210 requires a proportionate increase in the DC current source 205 that results in increased power consumption.
Third, skew between the two differential inputs INA and INB of the differential amplifier 200 may result. Skew may build between the inputs INA and INB because of a physical mismatch between the two transmission lines on which the signals travel. The skew distorts waveforms and progressively worsens as the differential signal propagates along the transmission line.
FIG. 3 shows an example of the distortion effect of skew on a differential signal A-B. The distortion effect progressively worsens as skew between the signals A and B increases as the differential signal travels along a transmission line. For example, no skew is evident when the signals are at the signal source 300. Some skew is apparent when the signals have traveled to the middle of the transmission line 305, and significant skew and distortion are shown when the signals have reached their destination 310. In addition, as skew builds, noise affects the two signals A and B unequally, thereby resulting in increased noise interference in the form of jitter.